Linear electromechanical actuators have been incorporated in aircraft over past years to operate critical flight elements, such as flight control surfaces and landing gear, while reducing fuel consumption due to the weight reduction obtained through the substitution of hydraulic and pneumatic systems for these lighter systems.
Critical flight elements mount redundant linear electromechanical actuators to ensure their operability upon electrical or mechanical failure of one of the linear electromechanical actuators. To this end, the failed linear electromechanical actuator must freely extend and follow the movement of the working linear electromechanical actuator that continues to operate the critical flight element.
Landing gears typically mount only one linear electromechanical actuator that must freely extend due to gravity upon electrical or mechanical failure of the linear electromechanical actuator.
Different solutions aimed at preventing failure of critical flight elements upon electrical or mechanical failure of one of the linear electromechanical actuators have been developed up to now.
A first solution consists of a linear electromechanical actuator with a screw-nut assembly engaged by means of a clutch to a gearbox driven by an electrical motor. Upon electrical or mechanical failure of the linear electromechanical actuator, actuation of the clutch disengages the screw-nut assembly from the gearbox, thus allowing free extension of the linear electromechanical actuator.
This solution does not prevent screw jamming, the main mechanical cause of failure of linear electromechanical actuators, as the disengagement occurs upstream of the screw-nut assembly.
Another solution consists of a pyrotechnic linear electromechanical actuator with a screw-nut assembly driven by an electric motor and a fuse-type piston engaged to the screw-nut assembly by retaining elements. Upon electrical or mechanical failure of the linear electromechanical actuator, explosive loads adjacent to the retaining elements are activated to destroy the retaining elements, which in turn allows free extension of the linear electromechanical actuator.
Any kind of electrical or mechanical failure will result in permanent disengagement of the fuse-type piston as the retaining elements have been destroyed. Therefore, the linear electromechanical actuator must be entirely mounted anew on the critical flight element after electrical or mechanical failure. This highly increases reparation costs.
A further problem with this solution results from the impossibility to carry out functional tests to ensure the correct behavior of the linear electromechanical actuator before installing it on the critical flight control element. Therefore, correct functionality of the linear electromechanical actuator must be entrusted.